Is reaction time variability in ADHD mainly at low frequencies?

Background: Intraindividual variability in reaction times (RT variability) has garnered increasing interest as an indicator of cognitive and neurobiological dysfunction in children with attention deficit hyperactivity disorder (ADHD). Recent theory and research has emphasized specific low-frequency patterns of RT variability. However, whether group differences are specific to low frequencies is not well examined. Method: Two studies are presented. The first is a quantitative review of seven previously published studies that have examined patterns of RT variability in ADHD. The second provides new data from a substantially larger sample of children than in prior studies (N_Control = 42; N_ADHD = 123). The children completed a choice RT task as part of a traditional go/stop task. Fast-Fourier transform analyses were applied to assess patterns of variability. Results: Quantitative review of previous studies indicated that children with ADHD demonstrate more low-frequency variability than non-ADHD controls (Hedge's g =.39; 95% CI:.16-.62), but an equivalent excess variability in a faster frequency comparison band (g =.36; 95% CI:.03-.69), with a trivial and nonsignificant difference between ESs in each band. New data replicated results of the quantitative review with nearly identical effects in the low-frequency (g =.39; 95% CI:.05-.75) and faster frequency comparison bands (g =.40; 95% CI:.04-.74) and no evidence of diagnosis × frequency interaction (p =.954). Conclusions: Results suggest that theories of RT variability in ADHD that focus on low-frequency variability will need to be modified to account for the presence of variability at a broader range of frequencies.